That when the skin is cut the blood spurts out is a proof that it is subject to pressure within the vessels. The Rev. Stephen Hales, the rector of Faringdon in Hampshire, was the first, at the beginning of the eighteenth century, to estimate what the degree of that pressure is in the living animal, and the animal he selected was the horse. He caused a mare to be tied down on her back, opened the main artery of the thigh, inserted into it a brass pipe the bore of which was 1/6 inch in diameter, and to this, by means of another brass pipe, which was accurately adapted to it, he fixed a glass tube, of nearly the same diameter, which was 9 feet in length. On untying the ligature previously placed on the artery, he observed the blood to rise in the tube 8 feet 3 inches perpendicular above the level of the left ventricle of the heart. This experiment was an original and a highly instructive one. It has often been repeated, not only in the horse, but in many other animals. The result of many observations has been to show that the pressure of the blood in the vessels is equal to that of a column of mercury 150 to 200 millimetres, or from 6 to 8 inches in height. But Hales pushed his experiment a step farther. He proceeded to investigate the effects of loss of blood on the general blood pressure. He measured the blood as it ran out of the artery, and after each quart of blood had escaped he refixed the glass tube to the artery " to see how much the force of the blood was abated ". This he repeated to the eighth quart, and then, its force being much lowered, he applied the glass tube after each pint had flowed out. He noted several remarkable circumstances. First, that as each quart of blood was removed the blood pressure sank considerably, but after the lapse of a minute, more or less, it again began to rise, and although it did not rise to its original level, yet it ultimately attained, on each occasion, a level higher than that to which it had previously fallen. This, there can be no reasonable doubt, was mainly due to the vessels accommodating themselves by virtue of their elasticity and their contractility to the reduced volume of their contents. Again, it was found that the decrease in the blood pressure was not strictly proportionable to the quantity of blood withdrawn; indeed, it sometimes rose above the level attained during the previous emission, which was probably due to variations in the degree of contraction and relaxation of the muscles in the walls of the vessels, and in the strength of the contraction of the heart. The blood pressure immediately rose when the animal strained its muscles to get loose - an effect that was due to the muscular contractions, especially those of the abdominal muscles, forcing much blood towards the heart. In this celebrated experiment about a quart of blood was lost in making the several trials, and Hales estimated that about 17 quarts were lost in all before the animal died. Taking into account the blood that was obtained from the vessels after death, he considered that 44 lbs. was a low estimate of the total quantity of blood in the horse.
The cause of the blood pressure is twofold. On the one hand the heart is always engaged in driving into the vessels, which are already distended, or more than full, an additional quantity of blood; and on the other the current of the blood experiences great resistance to its onward passage in the smaller vessels, owing to their reduced diameter and the great friction that it consequently experiences in traversing them. The blood pressure would be much greater than it is were it not that, owing to the large number of the capillary vessels, the channel is greatly increased, the united area of the capillaries having been estimated to be eight hundred times greater than that of the aorta.